Japanese Utility Model Publication No. 3-43,513 describes, in an oil pan structure including a deep bottom portion for storing oil to be sucked up by an oil pump and a shallow bottom portion shallower than the deep bottom portion close to the rotational components, an inclined surface formed in the shallow bottom portion lowering toward the deep bottom portion and lowering along the direction of rotation of the crank shaft so that the oil in the shallow bottom portion easily flows into the deep bottom portion.